The present invention relates to a surface acoustic wave device that operates within the VHF (Very High Frequency) and UHF (Ultra High Frequency) bands and, more particularly, to the electrode structure of a surface acoustic wave device.
Generally, a Surface Acoustic Wave (SAW) is excited and received by InterDigital Transducers (to be referred to as IDTs hereinafter) formed on a piezoelectric substrate and having interdigital electrodes. Japanese Patent Laid-Open No. 6-232687 (Reference 1) describes a technique for constituting a bandpass filter having wide pass band characteristics by two-dimensionally using two, longitudinal and transverse resonance modes, i.e., resonance caused by the mode distributed in the same direction as the propagating direction of the SAW (this mode will be referred to as the longitudinal mode hereinafter) and resonance caused by the mode distributed in a direction perpendicular to the propagating direction (this mode will be referred to as the transverse mode hereinafter).
FIG. 8 shows a surface acoustic wave filter described in Reference 1. Referring to FIG. 8, the surface acoustic wave filter is constituted by an input IDT 82, an output IDT 83, and grating reflectors 84 and 85 formed on a piezoelectric substrate 81. One half of the input IDT 82 in the direction perpendicular to the propagating direction of the surface acoustic wave serves as the IDT electrode of a resonator A, and the other half thereof serves as a dummy electrode. One half of the output IDT 83 in the direction perpendicular to the propagating direction of the surface acoustic wave serves as a dummy electrode, and the other half thereof serves as the IDT electrode of a resonator B.
In the conventional surface acoustic wave filter described above, the IDT electrodes of the resonators A and B are arranged adjacent to each other not to overlap in a direction (transverse direction) perpendicular to the propagating direction of the surface acoustic wave, in order to effect acoustic coupling of the transverse mode. Since the dummy electrodes are arranged in the same direction as the propagating direction of the surface acoustic wave, the lengths of the respective dummy electrodes in the propagating direction of the surface acoustic wave are equal to the lengths of the IDT electrodes constituting the resonators A and B, respectively. This hinders downsizing of the surface acoustic wave device.